TWINS energetic neutral atom observations of local‐time‐dependent ring current anisotropy

Abstract

Stereo energetic neutral atom (ENA) images from Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) are analyzed using a simple line‐of‐sight (LOS) mapping approximation, for two case studies: 6 April 2010 and 29 May 2010. A previous study first reported a global dawn‐dusk asymmetry favoring anisotropy on the duskside, observed by TWINS on 6 April. This paper extends and refines the previous analysis, and applies it to ENA data from 29 May, to find a second event containing the dawn‐dusk anisotropy asymmetry. Ion pitch angle distributions are retrieved using analytical expressions for anisotropy indices and their associated errors. The analysis is limited to intervals for which TWINS 1 and 2 have similar viewing geometries; i.e., the two spacecraft are at similar magnetic local time (MLT) but different latitudes, to sample two different but overlapping ranges of pitch angle. Application of the method is restricted to portions of the image for which the length of intersection of the LOS with the ENA source distribution is small. For each event, deduced pitch angle anisotropy is higher on the duskside than on the dawnside, a global asymmetry that persists for the hour‐long interval of favorable viewing geometry. The anisotropy generally decreases with energy from 1–30 keV. For 6 April the duskside anisotropy spectrum has a local peak at 12 keV, but exhibits an overall decrease over the measured energy range. For 29 May the duskside anisotropy falls monotonically with energy. Finding this previously unreported system‐level ring current attribute in two different events demonstrates that the global anisotropy is not a one‐time, unique occurrence, and also raises the question of how often and why this new phenomenon occurs. Further work is needed to determine the cause of the dawn‐dusk asymmetry in ion anisotropy. It is well known that multiple mechanisms, such as self‐consistent adiabatic ion drift, Coulomb collisions and charge exchange, influence the global ion pitch angle distribution. However, the degree to which these multiple mechanisms contribute specifically to the observed dawn‐dusk asymmetry will likely require direct comparison between the TWINS observations and simulations of these two events, including self‐consistent fields and realistic densities for the plasmasphere and neutral exosphere.